1. Field of the Invention
The present invention relates to a fluoropolymer seal material and method for making the same.
2. Description of the Prior Art
A phosphoric acid fuel cell stack is formed from a plurality of generally flat rectangular shaped fuel cells stacked one upon another. The cells use phosphoric acid as the electrolyte and may reach temperatures of 400.degree. F. The fuel or oxidant may be fed into each of the cells in the stack through openings in the sides of the stack by means of a manifold which covers at least a portion of the side of the stack and which must be sealed around the edge thereof against the surface of the stack to prevent leaking of the reactant gas. The surface of the stack against which the manifold must be sealed may have discontinuities of up to 0.050 inch due to tolerances resulting from the manufacture of the individual components in the stack and tolerances resulting from the assembly of the components.
From the foregoing, it is apparent that a seal material for this application must be compatible with phosphoric acid, be conformable to relatively large stack surface discontinuities, and have structural integrity, all at temperatures up to 400.degree. F. and for the life of the stack, which is desirably at least 40,000 hours. Also, the structural integrity or sealing ability must be maintained through numerous temperature cycles. Due to structural limitations of the stack and a desire to minimize the size, complexity and cost of stack hardware, it is also highly desirable that the seal material be able to effect a seal with low compressive forces.
No seal material on the market has been found to be suitable for this particular application. Elastomers such as ethylenepropylene rubber are suitable for temperatures below 325.degree. F.; however, at temperatures over 325.degree. F. they are corroded by phosphoric acid to an unacceptable extent and they have been found to lose their elastomeric properties due to air oxidation at these higher temperatures. Polytetrafluoroethylene rope type seal material presently on the market, such as Ready Seal.RTM.by Chemplast, Inc., Wayne, New Jersey, is very stiff. For example, it was attempted to use this material as the manifold sealant in the aforementioned fuel cell application, but a satisfactory seal could not be effected even with pressures between 450-500 psi. Grease compositions made with an oil base thickened with nonfibrillating PTFE, such as described in U.S. Pat. No. 3,493,513, do not have sufficient strength to maintain the seal integrity at high temperatures and throughout temperature cylces since the oil volatizes and weeps out, resulting in a drying out and cracking of this type of seal material.